Concentrating fluosilicic acid

ABSTRACT

A process for the concentration of aqueous fluosilicic acid solution is disclosed. In the process, a dilute aqueous fluosilicic acid solution is mixed with concentrated sulfuric acid in the presence of silica in a mixing zone to release a gaseous overhead of silicon tetrafluoride (formed by dehydration of the fluosilicic acid) and yield dilute sulfuric acid as bottoms; the gaseous overhead is passed to a scrubbing zone to effect reaction of the silicon tetrafluoride with dilute fluosilicic acid, and thereby yield a slurry of silica in concentrated, aqueous, fluosilicic acid solution. The silica and the concentrated acid are separated, and the silica can be sent to the mixing zone.

I United States Patent Parish et al. 5] Feb; 29, 1972 [54] CONCENTRATINGFLUOSILICIC ACID 3,233,969 2/1966 Heller et al. ..23/l82 [72] lnvemors:William R Parish; James C. Kelley, both 3,2l8,l25v ll/l965 Houston et al..23/l53 of Lakeland Primary Examiner-Edward Stern [73] ASSignBe:Wellmiill-Mld, In Attorney-John W. Bchringer, Eugene L. Bernard, MartinJ. Brown, James N. Dresser, W. Brown Morton, Jr., James T. [22] Flled'1970 Roberts, Malcolm L. Sutherland and Morton, Bernard, [2]] Appl. No.:17,590 Brown, Roberts & Sutherland [521 05.0. 23/153, 23/167, 23/182 v,[57] ABSTRACT 23/205 A process for the concentration of aqueousfluosilicic acid I 1 --C0l 7/00,C0lb 33/12, C011? 33/ solution isdisclosed. In the process, a dilute aqueous fluosil- 1 Field Search -2182 167 icic acid solution is mixed with concentrated sulfuric acid inthe presence of silica in a mixing zone to release a gaseous [56]defences Clad overhead of silicon tetrafluoride (formed by dehydrationof UNITED STATES PATENTS the t'luosilicic acid) and yield dilutesulfuric acid as bottoms; the gaseous overhead is passed to a scrubbingzone to effect 3,2 1 1/ 96 1 9), et 81 ..23/153 reaction of the silicontetrafluoride with dilute fluosilicic acid, 3,218,126 11/1965W|lk|nson.... 23/ 153 and thereby yield a slurry of silica inconcentrated, aqueous, 3,218,123 l1/1965 Klem 23/153 fluosilicic acidsolution.' The silica and the concentrated acid 1960347 5/ osswa-ld e123/153 are separated, and the silica can be sent to the mixing zone.2,456,509 12/1948 Hopkins, Jr. et al... ....23/l53 3,415,039 12/1968Rushton et a] ..23/l53 X 10 Claims, 1 Drawing Figure l4 '9 i. I l w a 1-12 l E SCRUBBING if uoum-soun HOLD-UP ZONE SEPARATION TANK a Z n he 2MIXING I ZONE PIIILNIEIIILm I972 [9 F l 1M6. I I42 I I I SCRUBBING I LIQUID- soLIII HOLD -UP 5 ZONE SEPARATION TANK a [II III 2 MIXING f Z ZONEINVENTORS WILLIAM R. PARISH JAMES c. KELLEY /6.4 Am 44mm ATTORNEYSCONCENTRATING FLUOSILICIC ACID Other applications, commonly ownedherewith, concerning processes of treating fluosilicic acid are US. Ser.No. 812,229, filed Apr. 15, 1969; US. Ser. No. 17,580, filed Mar. 9,1970; and US. Ser. No. 17,611,filed Mar. 9, 1970.

This invention relates to a process for the concentration of diluteaqueous solutions of fluosilicic acid by dehydration with concentratedsulfuric acid to prepare concentrated aqueous solutions of fluosilicicacid. Dilute sulfuric acid, suitable, for example, for digestingphosphate rock by the wet" process in the manufacture of phosphoricacid, is also produced.

The process of the present invention involves mixing strong sulfuricacid, including oleum, having a concentration generally of at leastabout 85 weight percent, say about 85 to 100 weight percent, preferablyabout 90 to 99 weight percent, with a dilute aqueous solution offluosilicic acid having a composition ranging normally from about toabout 30 weight percent H SiF and about 70 to 90 weight percent water,preferably about to weight percent H SiF] and about 75 to 85 percentwater.

The weight ratio of the acids in the mixing zone is such that thesulfuric acid bottoms has a concentration of at least about 65 weightpercent and advantageously about 70 to 95 weight percent. The weightratio on an anhydrous basis in the mixing zone is thus normally about 5to parts, preferably about 15 to 20 parts, sulfuric acid per part offluosilicic acid.

The mixing of the two acids is carried out in a mixing zone in thepresence of silica at a temperature of from about ambient, e.g., 80 F.,to about 300 F., preferably from about 100 to about 250 F., such thatthe fluosilicic acid is dehydrated as follows: H250J 1. 2H SiF +SiO3SiF,,+2H O The pressure in the mixing zone is sufficiently low to allowthe silicon tetrafluoride to evolve from the Zone as gaseous overheadand the diluted sulfuric acid to be removed from the zone as liquidbottoms. Atmospheric pressure is normally advantageous.

Dilute aqueous fluosilicic acid solutions containing about 10 to 30weight percent 1-1 SiF and about 70 and 90 weight percent water, andsuitable as feeds for the present process, are normally produced as abyproduct in phosphoric acid manufacture. The silica present in themixing zone can be supplied from the silica produced in the scrubbingzone, as explained below, and is generally added to the mixing zone as afilter cake of silica which is damp with concentrated fluosilicic acid.Reactive silica can also be supplied from other sources to the mixingzone and can, for example, be present in the fluosilicic acid feedsolution, for example by using as the feed solution that fluosilicicacid solution which is obtained from superphosphate scrubbers. Theamount of silica added to the mixing zone is at least aboutstoichiometric to the silica needed for the dehydration of thefluosilicic acid in accordance with the above reaction equation.

The solubility of the SiF, in the liquid mixture in the mixing zone is afunction of the temperature in the zone and of the concentration of thesulfuric acid solution therein. The lower the concentration of thesulfuric acid solution, the higher the temperature needed to drive thegaseous SiF from the liquid mixture. The temperature in the zone dependsmainly upon the inlet temperature of both acids, the heat of dilution ofthe sulfuric acid, and the heat of vaporization of the SiF gas. Thelatter (the heat of vaporization of the SiF, gas) tends tocounterbalance the heat of dilution of the sulfuric acid. Normally, itis not necessary to preheat the acids being introduced to the zone inorder to obtain satisfactory removal of SiF The gaseous overheadproduced in the dehydration is preferably anhydrous and preferablycontains essentially all of the silicon tetrafluoride produced from themixture of the sulfuric and fluosilicic acids and silica.

The fluosilicic acid is dehydrated during the time beginning with itsadmixture with the concentrated sulfuric acid and the sulfuric acid iscorrespondingly diluted. Silicon tetrafluoride and water are theproducts of the dehydration of fluosilicic acid. As noted before,silicon tetrafluoride exits the mixing zone as substantially anhydrous.gaseous overhead. The diluted sulfuric acid, diluted by the waterproduced in the dehydration reaction, exits the zone is liquid bottoms,for example having a sulfuric acid concentration of at least about 65,often about 70 to 95, weight percent. This concentration can becontrolled by adjusting the ratio of the sulfuric acid to thefluosilicic acid introduced to the mixing zone. Any excess silica in themixing zone also exits with the diluted sulfuric acid.

The amount of sulfuric acid in the bottoms from the mixing zone isadvantageously above about 65 weight percent, based on the combinedweight of water and sulfuric acid. Above about 65 weight percentsulfuric acid, the silica reaction with any HF which might be presentgives silicon tetrafluoride as the product, as follows: 1

2. 4llF-l-Si0 SiF +2H O However, below about 65 weight percent sulfuricacid, the reaction proceeds as follows:

3. 6HF SiO H SiF +2H O Thus, the amount of the sulfuric acid in thesulfuric acid bottoms should advantageously be above about 65 weightpercent, preferably above about 70 weight percent, so that silicontetrafluoride is produced. The silicon tetrafluoride is removed as agaseous overhead and then directly converted into fluosilicic acid inthe scrubbing or concentration zone described hereinafter.

The fluorine level present in the diluted sulfuric acid bottoms isnormally less than about 0.2 percent by weight. This low fluorine,diluted sulfuric acid bottoms product is useful in phosphoric acidmanufacture and sometimes may also be admixed with concentrated sulfuricacid to ,form a suitable sulfuric acid feed for the preset concentrationprocess.

The gaseous overhead containing essentially all of the silicontetrafluoride from the dehydration of the fluosilicic acid feed iscontacted with a dilute, aqueous, fluosilicic acid solution in thescrubbing zone. The fluosilicic acid wash or scrubbing solution mayadvantageously be of the same composition as previously described asbeing a useful feed to the mixing zone in the process of this invention.It is preferred that the wash solution be essentially silica-free. 1nthe scrubbing zone, the silicon tetrafluoride in the gaseous overhead iscontacted and reacted with water to form fluosilicic acid and hydratedsilica. The reactions in the scrubbing zone will be described in greaterdetail hereinafter.

The scrubbing zone serves to concentrate the fluosilicic acid content ofthe aqueous scrubbing medium in the hydration zone. Depending upon theconcentration of hydrogen fluoride in the gaseous overhead, theconcentration is accomplished by the occurrence of the followingreactions in varying degrees. Normally, however, there will be verylittle HF in the gaseous overhead, especially if there is an excess ofsilica over the stoichiometric amount required in the mixing zone.

4. 3SiF,+21-1 O ZH +SiO 5. 6HF+SiO H SiF +2H O 6. 4HF+SiO SiF.,+2H O 7.2HF+SiF.,- 1-1,,SiF

The scrubbing zone is normally maintained at a temperature of up toabout 150 F., e.g., from about ambient to about 150 F., preferably (andespecially where cooling water is available) about 50 to F. Thescrubbing zone can be supplied by any suitable gas-liquid contactingvessel as, for example, one or more spray towers which may containcooling means to maintain the temperature. Additionally, any silicontetrafluoride gases passing through the scrubbing zone unreacted may berouted to a second scrubbing zone for absorption in aqueous fluosilicicacid wash solution to reduce SiF, losses.

The primary reaction in the scrubbing zone is equation 4 above, whichyields a slurry of hydrated silica and concentrated fluosilicic acid.The slurry leaving the scrubbing zone may be treated to remove theprecipitated silica content. The separation of the silica from thefluosilicic acid product is accomplished in a silica separation zone,for example by centrifugation, filtration or decantation. The hydratedsilica cake can contain some of the concentrated fluosilicic acid but itis preferred that the separated cake have a I-l SiF ISiO ratio of lessthan about 4.8/1 by weight. The separated silica may be then used in thedehydration reaction in the mixing zone. Any residual concentratedfluosilicic acid which is present with the separated silica that is sentto the mixing zone is also dehydrated by the sulfuric acid in the zoneto produce silicon tetrafluoride gaseous overhead.

The fluosilicic acid solution from the silica separation zone is theconcentrated fluosilicic acid product of this invention. Theconcentrated product is at a temperature of up to about 150 F. andnormally about 50 to 120 F. A portion of the concentrated fluosilicicacid product can be recycled to the scrubbing zone to control theconcentration of hydrated silica within a desired range. This product isthen passed to a suitable storage zone. The concentrated fluosilicicacid solution prepared by the process of this invention has afluosilicic acid content ranging from about 20 to more than about 60percent by weight, depending upon the particular feed, scrubbing mediumused in the scrubbing zone and other process variables. The fluosilicicacid concentration of the concentrated solution product is always higherthan the fluosilicic acid concentration of the dilute solution feed.Normally, the products will have a fluosilicic acid content of fromabout 40 to 60 percent by weight.

The process of the instant invention can be more readily described byreference to the drawings in which the FIGURE sets forth a flowsheetillustrating the invention.

Referring to the FIGURE, fluosilicic acid, at a temperature of aboutambient up to about 250 F. and a concentration of from about 10 to 30weight percent l-l SiF is carried in line 1 to reactor 3 where it iscombined with 85 to l percent concentrated sulfuric acid introduced vialine 2 at a temperature of from about ambient up to about 240 F. Theweight ratio (anhydrous) of sulfuric acid to fluosilicic acid introducedto the reactor is about 5:1 to 30:1. The acids are mixed, heated ifdesired, and maintained at a temperature of up to about 300 F. Silicaremoved from the concentrated fluosilicic acid product, as describedbelow, enters the reactor via line 5. SiF, gas flashes off under theapproximately atmospheric pressure conditions maintained in the reactorand is removed via line 6. Dilute sulfuric acid of a concentration ofabout 70 to 95 weight percent, preferably about 80 to 85 weight percent,is removed through line 7 for ultimate recovery.

The gaseous overhead from the reactor 3 is passed into scrubbing zone 8,mixing at a temperature of from about ambient up to about 120 F., whereit is contacted with a wash liquid of a dilute, aqueous fluosilicic acidsolution supplied through line 9. The wash liquid is generally of thesame composition as the dilute fluosilicic acid solution feedstock inline 1.

The slurry of silica in concentrated fluosilicic acid solution, thelatter having a concentration of about 20 to 70, preferable about 40 to50, weight percent fluosilicic acid, is removed from the scrubbing zone8 through line 10 to the liquid-solid separation zone 11. Part of theslurry can be recycled from line 10 through line 12 into heat exchanger13 where it is cooled by indirect heat exchange with cooling waterentering heat exchanger 13 through line 14 and exiting through line 15.The cooled slurry reenters scrubbing zone 8 through line 16 from theheat exchanger 13 to assist in maintaining the temperature in thescrubbing zone. The silica separation zone 11 can contain suitablefiltering, centrifuging or decantation apparatus. The separated silicacan be passed through line 5 to the reactor 3 as the silica charge. Theseparated concentrated fluosilicic acid product can be conveyed toholdup tank 18 through line 17. Part of the concentrated fluosilicicacid product can be recycled from holdup tank 18 through line 19 to thescrubbing zone 8 to maintain the acid-silica concentration withindesired limits. The balance of the concentrated fluosilicic acid productcan be removed from holdup tank 18 through line 20 and sent to storage.

The concentration process of this invention is additionally advantageousfor integrated use in the preparation of superphosphate fertilizers. Forinstance, the diluted fluosilicic acid solutions useful herein as feedsmay be the byproduct of thepreparation of such fertilizers, while thesulfuric acid bottoms, after removal of hydrogen fluoride, are of suchconcentration that they are directly usable in the preparation ofphosphoric acid, of which substantial portions of the total yearlyproduction are used in the preparation of superphosphate fertilizers.

Also, the fluosilicic acid product solutions of this invention can be ofsuch high concentrations that considerable savings in transportationcosts are achieved. For instance, by increasing the concentration offluosilicic acid from 25 to 50 percent by weight in commercial grades offluosilicic acid solutions, substantial reductions in the water contentare achieved. As a result of these reductions, the transportation costper pound of fluorine can be reduced by about 50 percent. This reductionin transportation costs per pound of fluorine is significant in light ofthe large amounts of fluosilicic acid solutions used today in thefluoridation of water supplies.

The concentrated fluosilicic acid solutions produced by the process ofthis invention are also advantageous for direct use in the preparationof anhydrous hydrogen fluoride and can be used, for example, as thefeedstock in the process disclosed in copending application Ser. No.816,229, filed Apr. I5. 1969, in the names of William R. Parish, JamesC. Kelley, Albert Giovanetti and William A Lutz.

The following example serves to describe preferred procedures foraccomplishing the concentration process of this invention.

EXAMPLE The process flow for this example is illustrated by the Figure.By this embodiment of the invention, substantially pure (i.e., theprecipitated silica has been removed), concentrated fluosilicic acid isprepared. The process conditions are as follows.

Dilute fluosilicic acid, e.g., about 25 percent concentration by weight,in an amount of 273 pounds and at a temperature of about ambient, e.g.,about F., enters reactor 3 through line 1 while 6,268 pounds ofconcentrated, e.g., 93 percent by weight, H 50 at about F. enters thereactor through line 2. About 1,597 pounds of damp silica filter cakefrom the solid-liquid separation zone 11 enters the reactor through line5. The filter cake contains about pounds of silica and about 1,437pounds of a 50 weight percent aqueous solution of fluosilicic acid. 830pounds of SiF, gas is produced in reactor 3 and withdrawn via line 6.Dilute, e.g., about 80 percent by weight, sulfuric acid bottoms in anamount of about 7,306 pounds is removed from the reactor via line 7. Thedilute sulfuric acid bottoms contains a very minor amount, e.g., lessthan about 0.3 weight percent, H SiF The SiF, gas produced in reactor 3is contacted in the scrubbing zone 8 with 1,727 pounds of a side stream(via line 9) of the same dilute, e.g., 25 percent by weight, H SiFsolution that is sent to reactor 3 through line 1, thus producing 7,992pounds of a slurry of concentrated, e.g., 50 percent by weight aqueousfluosilicic acid solution and hydrated silica. The slurry contains about2 percent by weight (or 160 pounds) of hydrated silica (calculated asSiO The slurry is conveyed by line 10 to the silica separation zone 11.The filter cake, amounting to L597 pounds, which, as noted before,contains about 10 percent by weight silica, is conveyed from the silicaseparation zone 11 to the reactor 3 through line 5. The filtrate, 6,394pounds, which is the 50 percent concentrated fluosilicic acid solution,is conveyed via line 17 to the holdup tank 18.

5,434 pounds of the concentrated fluosilicic acid product is recycledvia line 19 to the scrubbing zone 8 and 960 pounds thereof is sent tostorage via line 20.

We claim:

l. A process for concentrating dilute, aqueous fluosilicic acid solutioncontaining about to 30 weight percent H SiF comprising:

i. mixing sulfuric acid having a concentration of at least about 85weight percent with a first portion of said dilute, aqueous fluosilicicacid solution in a mixing zone in the presence of silica to effectdehydration of the fluosilicic acid and dilution of the sulfuric acid,whereby silicon tetrafluoride is evolved as a gaseous overhead which isremoved, and the diluted sulfuric acid is removed as bottoms from thezone;

ii. contacting said silicon tetrafluoride gaseous overhead of (i) with asecond portion of said dilute, aqueous fluosilicic acid solution in ascrubbing zone to effect reaction between said silicon tetrafluoridegaseous overhead and the water in the dilute, fluosilicic acid solutionto form concentrated, aqueous fluosilicic acid and precipitated silica;

iii. separating the silica from the concentrated, aqueous fluosilicicacid solution and recycling the separated silica to step (i); and

iv. recovering concentrated aqueous fluosilicic acid solution productfrom the silica separation.

2. The process of claim 1 wherein the amount of silica employed in themixing zone is at least about stoichiometric to that required inaccordance with the reaction equation:

,zm m o 3. The process of claim 1 wherein the sulfuric acid is mixedwith the fluosilicic acid in step (i) in an anhydrous weight ratio ofsulfuric acid to fluosilicic acid of about 5 to 30:1 and sufficient toprovide the diluted sulfuric acid bottoms with at least about 65 percentsulfuric acid, based on the combined weight of water and sulfuric acid.

4. The process of claim 3 wherein step (i), prior to the mixing, thesulfuric acid and fluosilicic acid are each at a temperature of aboutambient up to about 300 F.

5. The process of claim 1 wherein the sulfuric acid employed in step (i)has a concentration of at least about 90 weight percent and the dilutefluosilicic acid solution contains about to weight percent H SiF 6. Theprocess of claim 1 wherein the sulfuric acid is mixed with thefluosilicic acid of step (i) in an anhydrous weight ratio of sulfuricacid to fluosilicic acid of about 5 to :1 and sufficient to provide thediluted sulfuric acid bottoms with at least about 70 percent sulfuricacid, based on the combined weight of water and sulfuric acid.

7. The process of claim 1 wherein part of the concentrated, aqueousfluosilicic acid solution product is recycled to the scrubbing zone.

8. The process of claim 1 wherein the scrubbing zone is maintained at atemperature of between about ambient and about 120 F.

9. The process of claim 8 wherein maintenance of the scrubbing zonetemperature is effected by circulating at least a part of the zonescontents through an indirect heat exchanger for cooling and thenreintroducing the cooled contents into the scrubbing zone.

10. A process for concentrating dilute, aqueous fluosilicic acidsolution containing about 15 to 25 weight percent l'l SiF comprising:

i. mixing sulfuric acid having a concentration of at least about weightpercent with a first portion of said dilute, aqueous fluosilicic acidsolution each of the sulfuric acid and fluosilicic acid being at atemperature prior to mixing of about ambient up to about 300 F., theanhydrous weight ratio of sulfuric acid to fluosilicic acid being fromabout 5 to 30:1 and sufficient to provide a diluted sulfuric acidbottoms having at least about 70 percent sulfuric acid, based on thecombined weight of water and sulfuric acid, in a mixing zone in thepresence of silica, in an amount at least about stoichiometric to thatrequired in ss rdanss. w tht stglbvx a isastt flirt 2H2SiF6 sio w ssiF,2520,

to effect dehydration of the fluosilicic acid and dilution of thesulfuric acid whereby silicon tetrafluoride is evolved as a gaseousoverhead which is removed, and the diluted sulfuric acid is removed asbottoms from the zone;

ii. contacting said silicon tetrafluoride gaseous overhead of (i) with asecond portion of said dilute, aqueous fluosilicic acid solution in ascrubbing zone maintained at a temperature of between about ambient andabout F., to effect reaction between said silicon tetrafluoride gaseousoverhead and the dilute fluosilicic acid solution to form concentrated,aqueous fluosilicic acid solution and precipitated silica, themaintenance of the temperature of the scrubbing zone being effected bycirculating at least part of the zones contents through an indirect heatexchanger for cooling and then reintroducing the cooled contents intothe scrubbing zone;

iii. separating the silica from the concentrated, aqueous fluosilicicacid solution and recycling the silica to step (i);

and 7 iv. recycling part of the concentrated, aqueous fluosilicic acidsolution from the silica separation to the scrubbing zone and recoveringthe remaining concentrated, aqueous fluosilicic acid solution productfrom the silica separation.

Patent: No. 3261454679 Dated February 9 97 lnventfls) William R. Parishand James C. Kelley It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 2, line 5 equation t should read-- BSiF 21120 a 2H SiF $10 Signedand sealed this 8th day of August 1972.

(SEAL) Attes't:

EDWARD PLFLE'JJCEEB JR ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM PC4050 (10-69) USCOMM-DC 60376-P69 h u.s. GOVERNMENTPRINTING ornc; 0969 0-;66.a34

2. The process of claim 1 wherein the amount of silica employed in themixing zone is at least about stoichiometric to that required inaccordance with the reaction equation: 2H2SiF6+SiO2 -> -> 3SiF4+2H2O. 3.The process of claim 1 wherein the sulfuric acid is mixed with thefluosilicic acid in step (i) in an anhydrous weight ratio of sulfuricacid to fluosilicic acid of about 5 to 30:1 and sufficient to providethe diluted sulfuric acid bottoms with at least about 65 percentsulfuric acid, based on the combined weight of water and sulfuric acid.4. The process of claim 3 wherein step (i), prior to the mixing, thesulfuric acid and fluosilicic acid are each at a temperature of aboutambient up to about 300* F.
 5. The process of claim 1 wherein thesulfuric acid employed in step (i) has a concentration of at least about90 weight percent and the dilute fluosilicic acid solution containsabout 15 to 25 weight percent H2SiF6.
 6. The process of claim 1 whereinthe sulfuric acid is mixed with the fluosilicic acid of step (i) in ananhydrous weight ratio of sulfuric acid to fluosilicic acid of about 5to 30:1 and sufficient to provide the diluted sulfuric acid bottoms withat least about 70 percent sulfuric acid, based on the combined weight ofwater and sulfuric acid.
 7. The process of claim 1 wherein part of theconcentrated, aqueous fluosilicic acid solution product is recycled tothe scrubbing zone.
 8. The process of claim 1 wherein the scrubbing zoneis maintained at a temperature of between about ambient and about 120*F.
 9. The process of claim 8 wherein maintenance of the scrubbing zonetemperature is effected by circulating at least a part of the zone''scontents through an indirect heat exchanger for cooling and thenreintroducing the cooled contents into the scrubbing zone.
 10. A processfor concentrating dilute, aqueous fluosilicic acid solution containingabout 15 to 25 weight percent H2SiF6 comprising: i. mixing sulfuric acidhaving a concentration of at least about 90 weight percent with a firstportion of said dilute, aqueous fluosilicic acid solution each of thesulfuric acid and fluosilicic acid being at a temperature prior tomixing of about ambient up to about 300* F., the anhydrous weight ratioof sulfuric acid to fluosilicic acid being from about 5 to 30:1 andsufficient to provide a diluted sulfuric acid bottoms having at leastabout 70 percent sulfuric acid, based on the combined weight of waterand sulfuric acid, in a mixing zone in the presence of silica, in anamount at least about stoichiometric to that required in accordance withthe following reaction equation: 2H2SiF6+SiO2 -> -> 3SiF4+2H2O, toeffect dehydration of the fluosilicic acid and dilution of the sulfuricacid whereby silicon tetrafluoride is evolved as a gaseous overheadwhich is removed, and the diluted sulfuric acid is removed as bottomsfrom the zone; ii. contacting said silicon tetrafluoride gaseousoverhead of (i) with a second portion of said dilute, aqueousfluosilicic acid solution in a scrubbing zone maintained at atemperature of between about ambient and about 120* F., to effectreaction between said silicon tetrafluoride gaseous overhead and thedilute fluosilicic acid solution to form concentrated, aqueousfluosilicic acid solution and precipitated silica, the maintenance ofthe temperature of the scrubbing zone being effected by circulating atleast part of the zone''s contents through an indirect heat exchangerfor cooling and then reintroducing the cooled contents into thescrubbing zone; iii. separating the silica from the concentrated,aqueous fluosilicic acid solution and recycling the silica to step (i);and iv. recycling part of the concentrated, aqueous fluosilicic acidsolution from the silica separation to the scrubbing zone and recoveringthe remaining concentrated, aqueous fluosilicic acid solution productfrom the silica separation.